1. Field of the Invention
This invention relates generally to the valves having a rotary valve head engagable with a valve seat, and more specifically to valves used to regulate the transmission of fluids in an oil or gas well, whether on the surface or at a subsurface location.
2. Description of the Prior Art
Valves for controlling the flow of fluids in a conduit by opening or closing the conduit commonly employ rotary manipulation of the valve head. Such valves suitable for maintaining a substantially fully open bore, without substantially restricting the fluid flow area through the valve, commonly employ a ball-type valve. Such valves can be used in a fluid transmission conduit carrying fluids to or away from a subterranean well producing oil or gas, or in the well head at the surface of the well, or at a subsurface location in the producing oil or gas well. For example, a test valve for producing a limited amount of fluid from a subterranean producing formation can be located at a subsurface location. A safety valve for automatically shutting-in a well in the event of an emergency is normally located at a subsurface location, especially in an offshore well, may also employ a ball-type valve.
Valves of this type can be rotated between the open and the closed positions in a number of different ways. For example, where the conduit containing a conventional ball valve is readily accessible, the ball valve head can be rotated about its axis by rotating a stem attached to the ball valve head. Appropriate lever means can be used to permit manual or mechanical rotation of the ball valve head. With the valve element located in an inaccessible location, such at a subsurface location in an oil or gas well, some other means must be provided to manipulate the valve head between its open and closed position. A typical means for actuating a rotary valve head located in an inaccessible location is use of an axially shiftable actuator or flow tube that can be moved towards and away from valve head in the direction of the axis of the fluid transmission conduit. An axially directed force is transmitted from the axially movable actuator flow tube to the rotatable ball valve head at a point offset from the axis of rotation of the valve head. For example, a pin engaging a ball valve head at a point spaced from the axis of rotation passing through the center of the valve head, perpendicular to the axis of the fluid transmission conduit, can transmit force from the axially reciprocal flow tube or actuator to the ball valve head at a point offset from the axis of rotation. Conversely, the rotatable valve head itself can be shifted in the axial direction relative to the fluid transmission conduit. Movement of an axially shiftable tubular member attached to a stem passing through the axis rotation of a ball valve element, moves the ball valve element axially within fluid transmission conduit or valve housing. A stationary element, such as pin, engaging the valve element at a point offset from the axis of rotation will then cause the valve element to be moved between the open and closed positions.
Sealing integrity is normally established in a conventional valve by engagement of the valve head with a cooperable valve seat. The valve seat normally comprises a circular or cylindrical member engagable with an abutting surface on the rotatable ball-type valve head. The valve seat normally includes a sealing surface or sealing element incorporated therein. For example, a resilient or elastomeric valve seal is often employed around the periphery of the cylindrical valve seat. This seal engages the spherical exterior of a ball-type valve head when the bore through the ball valve head is perpendicular to the axis of the valve housing. Some valve seats normally rely upon metal-to-metal contact between the valve seat and the ball valve head, although adequate sealing integrity can require excessive contact forces. It is not uncommon for conventional valves to employ metal-to-metal contact in conjunction with a resilient or elastomeric sealing element, so that the resilient sealing element is trapped between adjacent and supporting metal surfaces on the valve seat to prevent lateral extrusion of the resilient seal element when subjected high pressure. To insure that sealing integrity can be properly maintained, this outer spherical surface must be precisely machined, adding to the cost of the valve. Furthermore, the outer surface must have the proper surface finish, and that surface finish must be maintained if the valve is to function properly. Corrosion is often a problem in the environment in which ball valves are used, especially in conjunction with the production of oil and gas from a subterranean well. In the past it has been difficult to maintain the proper surface finish in the presence of corrosive fluids.
In order to avoid wear on the valve seat and the valve seal and to prevent sticking, some conventional ball valves employ an axially shiftable annular valve seat. For example, it is not uncommon to provide a valve seat with accompanying valve seal on an axially shiftable actuator or flow tube. This actuator or flow tube is normally spaced from the surface of the ball-type valve head during rotation between the opened and closed positions, and is only brought into engagement with the valve head surface when the valve has been shifted to the fully closed position.
Conventional ball valves, using conventional resilient or elastomeric sealing materials also have not been completely successful in preventing leakage of gases produced in subterranean wells or flowing in a surface conduit. Conventional elastomers are permeable and permit commonly encountered gases to pass directly through the body of the seal. If a metal-to-metal contact is to be maintained between the valve seat and the outer surface of the ball valve head to improve the sealing integrity in the presence of gases, integrity of the ball valve head surface contour and surface finish are even more critical.